In general, an optical system used in image pickup apparatuses, such as a digital camera and a video camera, is required to have a short total lens length (total optical length, length from the first lens surface on the object side to an image plane) and a compact optical system. However, as the total size of the optical system decreases, aberrations, especially chromatic aberration tends to increase and the optical performance tends to degrade. To properly correct chromatic aberration over the visible wavelength range in such an optical system for use in the image pickup apparatus, it is necessary to correct chromatic aberrations of four wavelengths of d-, g-, C-, and F-lines.
As a structure for correcting chromatic aberration, an optical element formed of a low-dispersion optical material and an optical element formed of a high-dispersion optical material are generally used in combination. However, since the high-dispersion optical material has a higher partial dispersion ratio for the g-line and the d-line than that of the low-dispersion optical material, if chromatic aberration between the F-line and the C-line is corrected by this structure, chromatic aberration of the g-line is apt to worsen.
Accordingly, a structure using a diffractive optical element is known as a method for correcting chromatic aberration of the g-line (PTL 1). A diffractive optical element has an anomalous partial dispersibility, has a small value of 0.8918 corresponding to the partial dispersion ratio for the g-line and the d-line, and is therefore effective for correction of chromatic aberration of the g-line. Further, since the absolute value of the value corresponding to the Abbe number is a small value of 3.45, only minute optical power is caused by diffraction, and chromatic aberration can be corrected with little influence on spherical aberration, coma aberration, astigmatism, etc. PTL 1 discloses a telephoto lens in which chromatic aberration is corrected by the use of a diffractive optical element.
In addition, as another structure for correcting chromatic aberration in the visible wavelength range, an optical system that corrects chromatic aberration with a diffractive optical element and a refractive optical element having an anomalous partial dispersibility has been proposed (PTL 2). PTL 2 discloses a zoom lens in which longitudinal chromatic aberration at the telephoto end and lateral chromatic aberration at the wide-angle end are corrected by arranging a diffractive optical element on an object side of an aperture stop and a refractive optical element on an image side of the aperture stop.
However, in the structure using the diffractive optical element as in PTL 1, chromatic aberrations of the d-, g-, C-, and F-lines can be relatively properly corrected, whereas it is difficult to simultaneously correct chromatic aberration of the g-line and chromatic aberration between the F-line and the C-line. This is because chromatic aberration caused in the diffractive optical element changes in proportion to the wavelength. Chromatic aberration caused in the diffractive optical element has a proportional relationship with the wavelength, and the slope of chromatic aberration of the wavelength between the F-line and the C-line is the same as the slope of chromatic aberration of the wavelength between the g-line and the F-line. In contrast, chromatic aberration caused in a general optical material tends to change in a curved manner with respect to the wavelength and to increase the slope of the curve as the wavelength decreases. Hence, the slope of chromatic aberration for the wavelength between the F-line and the C-line is different from the slope of chromatic aberration for the wavelength between the g-line and the F-line. For this reason, when chromatic aberration caused in the general optical material is corrected by the diffractive optical element, if an optical power necessary for correcting chromatic aberration between the g-line and the F-line is given to the diffractive optical element, chromatic aberration between the F-line and the C-line remains. Conversely, when an optical power necessary for correcting chromatic aberration between the F-line and the C-line is given to the diffractive optical element, chromatic aberration between the g-line and the F-line remains. Therefore, when the diffractive optical element is used, it is difficult to simultaneously correct chromatic aberration of the g-line and chromatic aberration between the F-line and the C-line.
The structure in PTL 2 corrects longitudinal chromatic aberration at the telephoto end by the diffractive optical element, and corrects lateral chromatic aberration at the wide-angle end by the refractive optical element. However, PTL 2 does not disclose a structure for properly correcting chromatic aberrations of the d-, g-, C-, and F-lines.
Accordingly, the present invention provides an optical element and an optical apparatus that can properly correct chromatic aberration over the entire visible wavelength range.